Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 120959
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
159 AM PST Tue Dec 12 2017

.DISCUSSION...

Short-term impacts:

 * Icy areas and poor visibility on roadways affected by nighttime
   and morning dense freezing fog/fog.

 * Degraded air quality due to stagnant air mass.

A stubborn upper ridge of high pressure continues to dominate the
weather for our area. Considering there are no major changes
expected to the weather pattern for the region in the next few days,
we relied upon the persistence model for the forecast. Recent polar
satellite imagery shows almost identical fog coverage early this
morning compared to 24 hours ago. Some roadways in the Rogue and
Illinois Valleys have become icy in the mornings due to freezing
fog, and this is especially true in shaded areas where some frost is
not melting much during the day and just accumulating. We
continue to advertise this impact, as well as poor visibility for
travelers, in Dense Fog and Freezing Fog Advisories that are valid
through the morning hours in West Side Valleys to include the
Rogue, Illinois, Umpqua, and Applegate.

The stagnant air mass with areas of nighttime and morning fog and
freezing fog will continue through Friday morning, and an Air
Stagnation Advisory is valid through that time. There are some
offshore, easterly winds that increase over the Cascades on
Thursday morning that may help improve mixing some Thursday, but
it`s a little too early to tell whether this will make much of a
difference. As higher- resolution models begin to populate for
that timeframe, we should have a better idea.

On Friday a cold front zips through the forecast area, and this will
improve mixing and should put an end to the stagnant air. GEFS mean
5000 foot temperatures lower by 12C in less than 24 hours. Despite a
sharp change in air mass, not much precipitation will accompany this
front, and what little there is should be focused over the coast
north of Cape Blanco, the Umpqua Basin, and Cascades north of Crater
Lake. The upper ridge then rebuilds behind this front but never
quite recovers to the full amplitude of the pre-frontal ridge.
With the weaker ridge in place, this could portend the potential for
wetter systems to move in during late December. Please see the
previous long-term discussion below for more details on this
potential.

Long Term...Friday, Dec 15th through Monday night, Dec 18th...
Model agreement is very good with the progression of a shortwave
trough through our forecast area for the first day of this long term
period, through 12Z on Saturday, Dec 16th. However, thereafter,
model agreement then quickly deteriorates regarding the amplitude of
a short wave ridge that will move in behind this trough, and with
details related to a subsequent trough after that early next week.
By next Wednesday the 20th, however, both models do agree on some
degree of troughing in the Pacific Northwest, albeit with sizable
significant weather differences. Technically speaking, the expected
changes in the pattern Friday through next Thursday (21st) are
generally consistent with phase 7 effects of the Madden Julian
Oscillation (MJO). This favors a period of precipitation in the
Pacific Northwest, though this round looks as if it will be light
(below daily averages) compared to climatological normals for mid-
December in our forecast area. Beyond the 21st, there are too many
competing variables to say what`s most likely to occur despite
current model leaning toward more high pressure ridging.

More specifically, the trough that`s expected to move in Friday is
likely to be enough to break and/ or significantly weaken the
persistent valley-mountain inversions over the forecast area. While
precipitation amounts look to be less than a tenth of an inch of
water with this trough, cooling aloft along with 30-50 knot 700MB
winds should provide enough mixing to freshen up the air for most,
if not all, areas.

For Saturday the 16th into Sunday the 17th, model guidance has
trended further north with the arrival of the next shortwave trough,
so the forecast has been trended in that direction. High pressure
ridging is likely to try to rebuild from the southwest during this
time period, but a faster flow aloft should result in better
vertical mixing across the forecast area- albeit with passing cloud
cover and some risk of precipitation, mainly over NW section of the
area.

The low presure trough expected early next week has had widely
varying details, in that there are several centers of competing low
pressure over the eastern Pacific as the trough arrives. If these
consolidate, the trough will be more significant than if they do
not. General indications are that this trough will be cold enough to
bring snowfall to pass levels or lower on or about Wednesday. How
much and for how long are details we`ll be attempting to grasp in
the coming days while this blocking high pressure ridge persists.
BTL


&&

.AVIATION...12/06Z TAF CYCLE...Areas of LIFR in fog and freezing fog
have started to fill the Umpqua and Rogue basins, and should impact
KRBG and KMFR throughout the night and into the morning. Some
clearing is expected during the late morning and early afternoon,
but some areas may not clear at all. Fog will make its return in the
evening.

VFR conditions will prevail elsewhere, including the East Side,
coast, and coastal waters. -BPN/BR-y

&&

.MARINE...Updated 200 AM PST Tuesday 12 Dec 2017...Relatively calm
will persist through the rest of the week. Moderate west swell will
build Thursday and subside Friday, and this may bring low end small
craft advisory seas south of Cape Blanco. A weak front will move
onshore Friday, followed by another weak front Saturday night.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Air Stagnation Advisory until 10 AM PST Friday for
     ORZ023-024-026-029>031.
     Dense Fog Advisory until 9 AM PST this morning for ORZ023.
     Freezing Fog Advisory until 9 AM PST this morning for
     ORZ024-026.

CA...None.

Pacific Coastal Waters...None.

$$

NSK/BTL/BPN



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